doing deco off the compy

[W Wilberforce]

Hello all,
Was doing some thinking and decided to waste you metal cycles instead.

I like to do 30-40m dives with my trusty twins and a single "deco gas" bottle, 70 or 80% typically.
I do all this without any pre-planning, just jumping in with an Apeks Quantum (two-mix wrist compy) and swimming back up when I have nearly too much deco
I worked out that going from air to 80 / 100 % O2 , the computer ~ halves remaining deco time) . Was pushing 20 -30 mins (actual) time at 10-3 metres in Scapa Flow)
Apparently this is 'wrong' and I should have the dive 'planned' before I jump in.

YET I simply cannot know what the compy will do past NDL limits, there is no decompresssion software model supplied with it, nor with the suunto's or any other compy maker I am aware

So what do other people do, and which model best follows the computers out there?

[Digger]

Ah, here is a man who has not done a deco proc course. Am I right? Yes, I think I am.

Call/PM Mark Powell. He will explain all this and a whole lot more. It will take a couple of days, and cost a few quid. But it will be worth it.

My answer to this question would take until past bedtime to explain. So I won't. Half answering the question is probably worse than not answering it at all, because I would misinform you, and I don't want to do that.

In short yes, you can get dive planners to do that. And yes, you can find out what your computer is goign to do after it gets into deco. But that really is the course content of a deco proc course. If you have already done a deco proc course go back and ask for your money back, because Mr Digger on the internet said so.

Digs.

[EdHornby]

I'll second what Digger said here - I went to Mark Powells lecture in nottingham and I was seriously impressed, he really knows his stuff.

[Tony Jay]

Originally Posted by spunkmire

Hello all,
Was doing some thinking and decided to waste you metal cycles instead.

I like to do 30-40m dives with my trusty twins and a single "deco gas" bottle, 70 or 80% typically.
I do all this without any pre-planning, just jumping in with an Apeks Quantum (two-mix wrist compy) and swimming back up when I have nearly too much deco
I worked out that going from air to 80 / 100 % O2 , the computer ~ halves remaining deco time) . Was pushing 20 -30 mins (actual) time at 10-3 metres in Scapa Flow)
Apparently this is 'wrong' and I should have the dive 'planned' before I jump in.

YET I simply cannot know what the compy will do past NDL limits, there is no decompresssion software model supplied with it, nor with the suunto's or any other compy maker I am aware

So what do other people do, and which model best follows the computers out there?

Essentially what you are doing is not 'wrong' as long as you have planned the what ifs.

Planning to me means

1. Knowing what time I'm going to surface by (or how else do other poeple know when to worry about me)
2. knowing I have enough gas for the dive
3. knowing I have enough gas if my deco gas blows.

So I plan the dive on my palmtop (jplan GF 30/85), so I can say what time I'm going to be out and check my gas calcs.

I write a run time from the tables.

I then jump in swim around do the deep stops off the plan, switch gasses at the appropriate depth and end up at my shallowest stop.
I then compare my plan to my two mix computer, depending on how I feel I do the plan deco or the computer deco (if cold I'll do the minimum !)

People may ask, who bother with the computer at all and just use a timer. The reason is that

1. for the second dive (20 m bimble etc) I can just strap the computer on jump in and swim around
2. If my plan changes underwater (can't find wreck / deco gas blown etc etc ) I have a computer to do all the bailout plans.

So I kind of jump in, swim around and wait until I'm into 20 min accelerated deco - its just I know when it is going to happen before it does !

Tony

[Ivon]

If your using 70-80% nitrox you should have covered gas/deco planning on your advanced nitrox course, if not I would be having a word with your instructor and asking why you haven't covered it.

[W Wilberforce]

Hello all,
thanks for your input.
I have done a TDI nitrox/ decompression course involving the use of gases up to pure O2. Throughout the course me and the instructor used decoplanner, we just plugged the numbers in and did the dive.
Yes, i do have a dive planner, I forked out for V-plan. I just cannot be bothered using it for sub-50m dives.
Also, is it OK to use 'nominal' conservatism on V-planner?
Will probably do what Tony does, just throw in some deep stops as the computer won't penalise me for that
Unfortunately I missed messr.s powells lectures so will have to read the book- any idea when it's out?

Oh by 'wrong' I mean
1. not managing my air supply prior to diving
2. having no idea how much deco and how long I will be (accurately) before hitting the bottom

So that will be a yes I guess!

[Tony Jay]

Originally Posted by spunkmire

Will probably do what Tony does, just throw in some deep stops as the computer won't penalise me for that

people may read that smiley in different ways !

How I think the smiley should be interpretted is "All computers will penalise you for coming up slower than their normal ascent speed" as they are ultimately restricting the shallow stop on nitrogen loading, which will increase if you come up slowly. In a Buelhman based model, deep stops increase your shallow stops and you ascent time (double whammy !). You get no time benefit from doing them, and in fact get penalised. An air ascent after a 40 m dive of 20 mins off a vytec could easily turn into a real ascent of 35 minutes if one starts doing deep stops at 20m or so.

A real problem of doing adhoc deep stops on a computer (as opposed to pre planned ones ) is you have no idea when you are going to the surface.

Tony

[Scuby]

As long as you're happy you can get yourself out of any situation you could get into, then carry on. If you're not confident that you can get out of the water after any possible failure, if you have no backup plan if your computer fails, if you don't know how much gas you need, if you regularly get out of the water with the bare minimum reserve left, or anything else that is a little "dodgy", then consider a full pre-dive plan.

Personally, I use my Vyper (single mix) for most diving, and pre-plan (using V-Planner) really only if i'm using trimix. For accelerated deco on nitrox I tend to find that planning on tables using both mixes gets me out just as quickly as using my Vyper on a single mix (even after throwing in some extra deep stops, etc), just because of the profile tracking that the computer can do. Recently i've started looking at v-planner plans before all dives (I have V-Planner for my phone, so easy to do), just to see what it thinks, but don't religiously follow them - just keep it in the back of my mind.

David

[EdHornby]

Deep stops would be a good idea, also bear in mind that deco isn't something to be minimised, you should do the right amount for the dive you do - I can understand the wish to get out of the water when it's cold and all you are doing is hanging at 5m, but a few minutes more to pad out to make sure you come out clean isn't going to hurt.....

as far as adding conservatism to the sofware you are using, again it's all about what you feel is the right amount of deco for the dive you're going to do - do some plans with the different levels of pad factors and look at the numbers and make some decisions on the facts in front of you. I'd say that a sub 50m is a committed dive !!!

I'm not sure where Mark Powell is up to with the publishers - but I'd go as far as saying I'll probably buy a proper copy of his book when it comes out (I have a PDF copy from the lecture) it's that good!

Please don't take any of the as 'this is the law' just my observations

most of all, dive safe

Ed

[Scuby]

Originally Posted by EdHornby

I'd say that a sub 50m is a committed dive !!!

Guessing he means less than 50m, rather than below 50m...